__init__(): Be sure to set the default type to message/rfc822.
diff --git a/Lib/email/MIMEMessage.py b/Lib/email/MIMEMessage.py
index 8fa86cb..2042dd9 100644
--- a/Lib/email/MIMEMessage.py
+++ b/Lib/email/MIMEMessage.py
@@ -28,3 +28,5 @@
         # It's convenient to use this base class method.  We need to do it
         # this way or we'll get an exception
         Message.Message.attach(self, _msg)
+        # And be sure our default type is set correctly
+        self.set_default_type('message/rfc822')